The present disclosure relates to technology for non-volatile storage.
Semiconductor memory is used in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Some non-volatile memory store information in a charge storage region that is insulated from a channel region in a semiconductor substrate. As one example, a floating gate is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
Some non-volatile memory utilizes a charge trapping layer to store information. One such example has an oxide-nitride-oxide (ONO) region, in which the nitride (e.g., SiN) serves as a charge trapping layer to store information. When such a memory cell is programmed, electrons are stored in the charge trapping layer.
Non-volatile memory could have a 2D architecture or a 3D architecture. Recently, ultra-high density storage devices have been proposed using a 3D stacked memory structure having strings of memory cells. One such storage device is sometimes referred to as a Bit Cost Scalable (BiCS) architecture. For example, a 3D NAND stacked memory device can be formed from an array of alternating conductor and insulator layers. A memory hole is drilled in the layers to define many memory layers simultaneously. A NAND string is then formed by filling the memory hole with appropriate materials. A straight NAND string extends in one memory hole, while a pipe- or U-shaped NAND string (P-BiCS) includes a pair of vertical columns of memory cells which extend in two memory holes and which are joined by a pipe connection. Control gates of the memory cells are provided by the conductor layers.
Prior to programming certain non-volatile memory devices, such as a NAND flash memory device, the memory cells are erased. The erase operation removes electrons from the floating gate, for some devices. For some devices, the erase operation removes electrons from the charge trapping layer. Thus, the erase may lower the threshold voltage of the memory cell.
Programming of the memory cells may be achieved by applying a program voltage to the control gate to raise the threshold voltage of the memory cell. Typically, a sensing operation, referred to as program verify, is performed after the program voltage is applied in order to determine whether the memory cell has reached its intended threshold voltage. This process can be repeating in a number of program loops. Memory cells can be locked out from further programming after they reach their target threshold voltage to prevent over-programming.